Why will a background image not show up?
Background images are only visible in Camera View or in the Preset Ortho views along an axis.


Numpad 0 for camera view
Numpad 5 : shifts between ortho and perspective view
Numpad 1 : front view
Numpad 3 : right view
Numpad 7 : top view
Ctrl+Numpad 1 : back view
Ctrl+Numpad 3 : left view
Ctrl+Numpad 7 : bottom view